Allocate the channel arrays dynamically

Allocate the channel arrays dynamically

This avoids having to allocate space for all theoretical channels
if most of them aren't used. This drops the size of the full
RTMP struct from over 1200 KB to 16 KB (on 64 bit), and as long as
only channels with a low number are used, the amount of total
allocated memory stays far below what it was before.